Wellbore operations using controlled variable density fluid

ABSTRACT

Fluid systems may contain elements to provide changes in bulk fluid density in response to various environmental conditions. One environmental driver to the variable density is pressure; other environmental drivers include, but are not limited to, temperature or changes in chemistry. The variable density of the fluid is beneficial for controlling sub-surface pressures within desirable pore pressure and fracture gradient envelopes. The variability of fluid density permits construction and operation of a wellbore with much longer hole sections than when using conventional single gradient fluids.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional from U.S. patent application Ser. No.12/353,587 filed Jan. 14, 2009, issued as U.S. Pat. No. ______ on______, which is a divisional from U.S. patent application Ser. No.11/155,172 filed Jun. 17, 2005, now abandoned, which in turn claims thebenefit of U.S. provisional application No. 60/582,687 filed Jun. 24,2004, all of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to wellbore operations fluids, and moreparticularly relates, in one embodiment, to wellbore operations fluidshaving variable density.

BACKGROUND

Prior variable density drilling fluids primarily concerned the use of ahighly compressible gas (e.g. air or nitrogen) as a free phase in thefluid. Limited, if any, efforts are made during conventional air, mistor foam drilling to control the expandability of the bulk fluid or toadjust or engineer the compressibility of the fluid other than managingthe ratio of air or other gas to the fluid. Other proposals to employ avirtual multiple gradient fluid include so-called dual gradientdrilling. This method would use two columns of different density fluids.One column would be essentially static, while the second fluid densityis circulated below the seafloor. During drilling the vertical height ofthe in well bore column would change as the well is deepened and theresulting bulk average fluid density along the wellbore would thus varywith depth.

Typical “single gradient” fluids used today include multiple components(base fluid, various solids and additives). The density of the basefluids is known to vary with temperature and to some degree withpressure. While these density changes are often accounted for during themathematical modeling of the fluid pressures in the wellbore, thedensity changes resulting from this behavior is not sufficient to changethe design of the wellbore with respect to pore and fracture pressureprofiles, as well as position and number of casing strings. No effort isknown to be made to intentionally modify the compressibility (density)of classic drilling fluids.

Thus, it is desirable if a true variable density fluid were devisedwhere the properties of the fluid could be designed to fit therequirements of the wellbore operation and the subterranean formationsbeing drilled. It would also be desirable if a variable density fluidcomposition could be devised that is recirculatable on the current welland/or reused on a second or subsequent well.

SUMMARY

Variable density fluids are provided for wellbore operations.

A method of drilling a wellbore using a variable density fluid isprovided where the density of the fluid changes by design as a functionof external parameters that vary along the depth or length of the well.

A method is described of drilling a wellbore with a reusable, variabledensity fluid that permits construction and operation of a wellbore withlonger hole sections than when using conventional single gradientfluids.

There is also provided, in one form, a method for a fluid of variabledensity that includes a base fluid and a plurality of elements thatchange their volume/weight ratio in response to a condition of pressure,temperature, and/or chemical composition of the base fluid. The variabledensity fluids herein have unit densities that can be deliberatelychanged as contrasted with existing fluids where the bulk fluid densitychanges only slightly in response to temperature and/or pressure.

There is further provided in another non-limiting embodiment a method ofconstructing a wellbore that includes drilling a wellbore using awellbore operation fluid within the wellbore. The wellbore operationfluid is subjected to a condition, where the density of the fluidchanges in response to a condition such as pressure, temperature, and/orchemical composition of the base fluid. The fluid includes a base fluid,and a plurality of elements that change their volume/weight ratio inresponse to the condition.

There is additionally provided a method of improving the lift of aproduced fluid that involves injecting into the produced fluid at asubsurface point an effective amount of a plurality of elements thatchange their volume/weight ratio in response to a condition that may bepressure, temperature, and/or chemical composition of the produced fluidto increase the lift thereof.

There is also provided an element that changes its volume/weight ratioin response to a condition that includes a non-deformable core, acompliant skin surrounding the core, and at least one gas-filled spacebetween the non-deformable core and the compliant skin, where thecondition includes pressure, temperature, and/or chemical composition ofthe base fluid.

Additionally provided in another non-limiting embodiment is an elementthat changes its volume/weight ratio in response to a condition, wherethe element includes a non-deformable pseudo-porous body; and at leastone closed cell compliant component, where the condition includespressure, temperature, and/or chemical composition of the base fluid.

Still further provided is a non-limiting embodiment that involves ahollow rigid external shell having at least one cavity therein and atleast one opening into the cavity and an inner material within thecavity that changes its volume/weight ratio in response to a condition,where the condition includes pressure, temperature, and/or chemicalcomposition of the base fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are schematic, cross-sectional illustrations of variousstates of an element of one embodiment, where FIG. 1A is an element ofthe variable density wellbore operation fluid in its fully expandedstate of the largest volume/weight ratio, FIG. 1B is the element of FIG.1A in an intermediate state, and FIG. 1C is the element of FIGS. 1A and1B in its fully contracted state of the smallest volume/weight ratio;

FIGS. 2A-2C are schematic, cross-sectional illustrations of variousstates of another non-limiting embodiment of an element, where FIG. 2Ais an element of the variable density wellbore operation fluid in itsfully expanded state of the largest volume/weight ratio, FIG. 2B is theelement of FIG. 2A in an intermediate state, and FIG. 2C is the elementof FIGS. 2A and 2B in its fully contracted state of the smallestvolume/weight ratio;

FIG. 3A is a schematic, cross-sectional illustration of an alternateembodiment showing a different element in an intermediate state betweenfull expansion and full contraction;

FIG. 3B is a schematic, cross-sectional illustration of the alternateembodiment of FIG. 3A illustrating the element in a state of full orcomplete contraction;

FIG. 4A is a schematic, cross-sectional illustration of yet anothernon-limiting embodiment showing a pseudoporous particle (e.g. carbide)with volumes of a cellular elastomer in a relatively contracted state;

FIG. 4B is a schematic, cross-sectional illustration showing thepseudo-porous carbide particle of FIG. 4A where the cellular elastomeris in a relatively expanded state where the cells are enlarged ascompared with FIG. 4B.

FIG. 5A is a schematic, three-quarters view of one more embodimentshowing a hollow or porous shell with an inner, volume-changingmaterial;

FIG. 5B is a schematic, cross-sectional illustration of the FIG. 5Aembodiment where the inner, volume-changing material is in a relativelycontracted state;

FIG. 5C is a schematic, cross-sectional illustration of the FIG. 5Bembodiment where the inner, volume-changing material is in a relativelyexpanded state;

FIG. 6A is a schematic graph of fluid bulk density as a function ofwellbore depth for a fluid that becomes more dense with depth, pressure,etc.; and

FIG. 6B is a schematic graph of fluid bulk density as a function ofwellbore depth for a fluid that becomes less dense with depth, pressure,etc.

It will be appreciated that the drawings are schematic illustrations andare not necessarily to scale, and are employed to further illuminate theinvention which is not limited to the particular, specific embodimentsshown.

DETAILED DESCRIPTION

During the drilling of a wellbore, fluids are used to control thepressure of exposed formations. Fluid pressures developed must besufficiently high to prevent flow of formation pore fluids into thewellbore while also be sufficiently low to prevent fracturing or lostreturns to formations elsewhere in the open hole section. This processis referred to as staying within the pore-frac window. A consequence ofefforts to stay within the pore-frac window is the setting and cementingof strings of casing or liner. That is, the conventional solution tostaying within the pore-frac window is to set and cement pipe and thenchange the mud weight in the next section of the borehole. Alternativesolutions encompass, but are not necessarily limited to, extending orpushing the open hole length to and beyond a “safe” limit prior tosetting and cementing pipe, including temporary strengthening or liningof the hole and intentional plugging of any potential flow zones. Theselast two concepts seek to function or perform as partial or temporarylinings.

An alternative to efforts to modify or change the wellbore strengthand/or fluid pressure communication is to change the profile of thehydrostatic pressure developed by the fluid column so as to stay withthe pore-frac window and allow longer hole sections. In general, twomethods are currently proposed in which the goal is to develop asubstantially variable fluid gradient along the length of the open holesection. The two methods involve the use of air or nitrogen in the fluidand so-called “dual gradient” drilling. These methods are furtherdescribed in the Background.

Other methods have been discovered to modify the fluid density withinthe wellbore. Fluids have been discovered which vary their density in apre-planned and pre-engineered, designed manner for the beneficial useof extending the length of hole sections before setting casings orliners. Initially, consider a gas filled sphere or element 10 with acompliant or elastic skin 12, at least one gas-filled void 14 and anon-deformable core 16 as seen in FIGS. 1A-1C. The compliant skin 12 issuch that a maximum expansion limit can be reached where the gas willnot be allowed to continue expanding the gross sphere size, as shown inFIG. 1A. In one non-limiting embodiment the compliant skin may be anelastomeric polymer or rubber. The elements 10 are solids and areinsoluble in the base fluid. The result of this construction is a sphere10 in which the volume responds to external pressure. At some lowpressure, the volume has reached a maximum and further reductions ofexternal pressure will not result in an additional expansion (FIG. 1A).Also, at some upper pressure, the gas (or skin 12) will have collapsedupon the internal sphere core 16 and no longer shrinks with additionalexternal pressure (FIG. 1C). The resulting spheres thus have a range ofvariable size determined by external pressure; note that FIG. 1B showselement or sphere 10 in an intermediate state between the maximumexpansion state of FIG. 1A and the minimum contraction state of FIG. 1C.

It is possible to envision an alternate embodiment having nonon-deformable core 16, such as the embodiment shown in FIGS. 2A-2C,where FIG. 2A shows a gas filled sphere or element 20 with a compliantor elastomeric skin 22 surrounding at least one gas-filled void 24 inits maximum expansion state. FIGS. 2B and 2C shown sphere 20 inintermediate, and minimum contraction states, respectively.

It should be kept in mind that a small increase in linear dimension ofthe element, such as an element 20, is cubed upon volumetric expansion.For instance, if the increase in internal diameter of the void 24 ofelement 20 from FIG. 2C to FIG. 2A is 2.5/1, the volumetric increase isover 15/1. Stated another way, the elements have an average fullyexpanded state or size and an average fully or completely contractedstate or size, where the volume ratio of the average expanded state toaverage contracted state is at least 2.5, alternatively the volume ratiois at least 5, in another non-limiting embodiment the volume ratio is atleast 10, in a different, non-restrictive version the volume ratio is atleast 5, or alternatively the volume ratio is at least 50.

Through appropriate engineering design of the relative amounts of thespheres 10 or 20 within a drilling fluid, it is possible to have adrilling fluid that significantly changes density in response to localpressure. Other parameters that may influence the amount of densityvariation include the base material density of the structural elements(12, 14 and 16; or 22 and 24) and the nature of the expanding or elasticmaterial (12 or 22) of the sphere (10 or 20, respectively). Additionalimportant parameters in the design of the controlled compressibilityfluid include the pumpability of the resulting fluid and interactionwith other solid elements in the fluid such as drill cuttings, orspecial mud solids for filtration and/or viscosity control, e.g. gels.

It is expected that in some non-restrictive embodiments the separationof the spheres or elements from the fluid must be addressed. In aparticular embodiment, the separation of mud solids from drilling mudsand particles from other drilling fluids is an issue that typically ismanaged with active circulation and/or stirring of the mud pits or othercontainers. In the actual bore hole, the viscosity of the fluid phasehas to be great enough that the combination of the buoyancy effect andviscosity effectively float the particle within the mud or fluid. Thiscauses the mud or fluid to “hold” the weight or buoyancy of the particleand therein allows the composite density of the mud or fluid to reflectthe nature of the combination of many elements, including the spheresand elements described herein.

More specifically the unit densities of the drilling muds herein arelikely to be within a wider range than in typical muds, and apparentdensities will change as a function of pressure, in one non-limitingembodiment. In some non-restrictive applications, the spheres orelements may tend to drift or sink at downhole pressures, and/or may tryto float or rise in the mud tanks at the surface. These effects mayaffect the way that mud systems are managed, but they are not expectedto be limitations on the practicality of the concept herein.

Multiple embodiments of this controlled variable density fluid areenvisioned. Described above with respect to FIGS. 1 and 2, the elementis given as a gas-filled sphere, 10 or 20, respectively. However, theelement shape is not required to be spherical nor gas filled. In theembodiment above, the driver for the element expansion is a gaspressure. The gas does not need to be conventional air or nitrogen andmay be composed of material with much higher liquification pressures dueto the relatively high pressure encountered in sub-surface wellboreoperations. Non-limiting examples of such a gas or fluid include, butare not limited to natural gases (e.g. oilfield gas) which may beselected to have a wide range of compressibility behaviors, e.g. a widerange of Z factors.

Also, the expansive driver does not need to be a gas phase, it can alsobe a spring link or spring-like component constructed on a micro-basis.Shown in FIGS. 3A and 3B is a schematic illustration of a cross-sectionof element 30 having a schematically illustrated spring component 32within the element body 34, where the spring component 32 is attached toa piston 36 that travels between upper rail condition 37 and lower railcondition 39. It is not necessary that spring component 32 be a classiccoil, leaf or wafer spring as long as it exhibits spring-like behavior,i.e. can move into an expanded configuration or contracted positionunder an outside influence or condition. FIG. 3A illustrates element 30in an intermediate position, where the spring component 32 is partiallyexpanded and piston 36 is approximately half-way between the upper railcondition 37 and lower rail condition 39. Compare FIG. 3B with FIG. 3Awhere in FIG. 3B the spring component 32 is fully contracted and piston36 is against upper rail condition 37. The volume of the element 30 isdecreased by ΔV as fluid 38 flows into the body 34 of element 30 in thedirection of the arrow and the total decrease in volume in the systemfrom the sum of the ΔVs of each element 30 is Total ΔV. Elements such asschematically illustrated in FIGS. 3A and 3B may be micro- ornanomanufactured using current and future techniques.

FIGS. 3A and 3B are also helpful tools to mathematically model thevolume change ratios needed to estimate or calculate the value benefitin particular applications of the variable density fluids, for instancethe changing mud density in a wellbore.

In one non-limiting embodiment, a variable density fluid containingelements such as elements 30 would behave on the surface as a 10 lb (10lbs/gal or ppg) density fluid, that is at 0 feet of depth and 0 psipressure (atmospheric pressure) under lower rail conditions, whereas thesame fluid containing elements 30 may behave as a 24 ppg fluid at adepth of 14,000 feet and a pressure of 12,400 psi under upper railconditions, where the composite average between the two rail conditionsis about 17 ppg. A schematic graph of how the density of such a fluidwould change is shown in FIG. 6A. That is, FIG. 6A is a representationof densification with depth, pressure, and/or other factors.

In an alternative non-limiting embodiment, the method could be practicedin such a way, and the elements may be designed in such a way that thefluid bulk density decreases with depth, pressure, and/or other factors.That is, such a fluid would become lighter or there would be“undensification” or reverse densification in response to particularconditions, as schematically illustrated in FIG. 6B.

It should also be understood that the base fluid density does not haveto initially be within the range of the desired bulk density variationof the variable density fluid. The initial base fluid density may bevery light or very dense. For instance, in the non-limiting case wherethe base fluid was very light or “super-light”, e.g. a gas fluid or afoam, the variable density elements would always increase the variablebulk average density of the resulting fluid to a variable value greaterthan that of just the base fluid. Non-limiting examples would includethe use of foam or air as drilling fluids.

On the other hand, the initial base fluid may be very dense, in anon-restrictive example a “super-dense” or “super-heavy” fluid, e.g.where the base fluid has a density at or higher than the greatestincremental density desired or designed. In this case, the elementswould always lighten or decrease the base fluid density. Non-limitingexamples include mineral separation fluids with specific gravitiesapproaching 4 (e.g. densities of about 35 ppg or more), such as aqueoustungstenates.

Yet another configuration is to intentionally use base fluids (or blendsof fluids) with high compressibilities to accomplish a portion or all ofthe needed fluid density variation.

The initial application for this controlled variable density is in theconstruction of wellbores. This includes the initial open hole drillingwith drilling muds. The process of placing cement around casing andliner strings is also limited to some degree by the density of thepumped cement. The variable density spheres or elements may also beadded to cement to provide improved cement placement characteristics andopportunities. The variable density elements are also expected to findutility in other sealants or sealing materials besides cements,including, but not necessarily limited to, epoxies, expansive liquids,gels, dehydrated slurries, and materials that form temporary orpermanent partial or complete barriers, and the like. Other applicationsfor fluids with variable densities may also be imagined.

One feature is that the element changes in response to a localenvironmental physical parameter or condition that may vary along thelength or depth or distance of the wellbore. The element is dependentupon something else that naturally changes or that the operator changes.The changes may be in the environment, in the base fluid or both. Therate at which volume will change in response to a condition such aspressure, temperature, chemical composition, or other factor can also bedesigned and determined in advance, based on the parameters discussedabove, that is, including, but not necessarily limited to the materialcompositions of the elements, the physical dimensions of the elements,the properties and physical composition of the base fluid which carrythe elements. In some cases, the volume may drop sharply with pressureand in other cases more gradually. Instances or circumstances where theelements may change their volume/weight ratio relative to chemicalcomposition of the environment include, but are not necessarily limitedto, changes in the brine salt concentration, changes in pH, electricalproperties of the fluid, and the like and combinations thereof. Theelements of the fluids herein may also change their volume/weight ratioin response to other properties of the fluid, including, but notnecessarily limited to, electrical properties of the fluid, magneticfield, radiation (natural or induced), and the like and combinationsthereof. The response to the local environment could be any of the manymechanisms mentioned herein or suggested by others, downhole orotherwise, for control of water influx via swelling or plugging of porespaces.

Yet another embodiment is schematically illustrated in cross-section inFIGS. 4A and 4B. FIG. 4A illustrates an element 40 having apseudo-porous body 42 that is essentially non-deformable, for instancesilicon carbide dust or metals or metal oxides. An important goal to theselection of the pseudo-porous body is to achieve a specific gravitycore so the collapsed element behaves or acts on the system like aweighting material. These bodies 42 would have at least one cellularcompliant or elastomeric component or material 44 thereon, where theindividual cells 46 would not communicate with one another. Forinstance, in the case that material 44 is a foam, the foam would be aclosed-cell elastic foam, rather than an open cell foam. Optionally, thecells may be filled with a gas, such as nitrogen, air, a noble gas, etc.Although cells 46 are illustrated as spheres, it will be understood thatthey need not necessarily be spherical, but may be any volumetric shape.In FIG. 4A, the cellular compliant component 44 is collapsed orcontracted and cells 46 are relatively small, whereas in FIG. 4B, thecellular compliant component 44 is expanded or enlarged and the cells 46are relatively large. In this embodiment, notice that the mean averagemaximum size of the element 40 essentially does not change.

Still another embodiment is depicted in FIG. 5A which shows element 50having a hollow or porous rigid external shell 52, which in very roughanalogy may or may not resemble a “whiffle” ball, with at least onecavity 58 and one or more orifices or openings 54 therein between theexterior and the cavity 58. Outer structural shell 52 limits theexpansion of an inner material 56 within the cavity 58 of shell 52. Theinner material or “kernel” 56 provides the minimum expansion limit (FIG.5B), while the external shell 52 defines the maximum expansion limit(FIG. 5C).

As noted, pumpability of the elements and the base fluid is a parameterto be designed herein, and it will be understood that the elements willbe very small. In one non-limiting embodiment, the average particle size(largest dimension) of the elements is about 100 microns or less,alternatively about 75 microns or less, and in still anothernon-restrictive embodiment about 50 microns or less. In embodimentsshown such as those in FIGS. 1 and 2 where the average element diameterchanges, the initial average element diameter may be about 100 micronsor less and contract or shrink to within the range of about 30-50microns or less. Alternatively, in another non-restrictive form, theinitial average element diameter may be about 70 microns or less andcontract or shrink to within the range of about 25-35 microns or less.In one non-limiting embodiment, the sizes are designed to generally passthrough a shaker screen and still be the approximate size of current orconventional barite grind (>325 mesh; >44 microns) when collapseddownhole. In another non-limiting embodiment these average particlesizes may be for the average contracted state size, in the case wherethe elements have an average contracted state size (or contractedcharacteristic dimension) and an average expanded state size (orexpanded characteristic dimension).

It is expected that in some non-limiting embodiments certain of theelements will fail, that is will be stuck at an expanded or contractedstate, that is, lose their ability to change their volume/weight ratio.Thus, it may be useful to determine a “failed case” density and size topermit simplified deletion/extraction of any failed elements from thesurface mud system. This would be similar to or analogous to bariteweighting agent recovery via a mud cleaner, but in this embodiment woulddiscard the broken or failed elements.

Manufacture of the elements for the fluids, such as elements 30 of FIG.3 can be performed using micromanufacturing or nanomanufacturingtechniques, as noted. Elements 10 and 20 as seen in FIGS. 1 and 2,respectively may be produced by known and future microencapsulationmethods. Elements 50 shown in FIGS. 5A-5C could be made bymicroencapsulation, micromanufacturing and/or nanomanufacturingprocesses. Elements 40 such as shown schematically in FIGS. 4A and 4Bcould be produced by grinding or pulverizing tungsten carbide, siliconcarbide, other dense, carbide-like element or other pseudo-porousmaterials, infusing the particles with a dense, but microcellularpolymeric elastomer, and then cryogenically grinding the elastomer downto essentially the initial size of the particles. Other possiblematerials for the non-deformable pseudo-porous materials besides carbideinclude, but are not necessarily limited to, silicon oxide (glass orsand), nanocarbon structures such as nanotubes or Buckminster fullerenes(buckyballs), and the like.

It should also be appreciated that the wellbore operation fluids hereinmay contain more than one kind of element, that is, more than oneelement embodiment may be employed at once. Indeed, the use of differenttypes of elements that change their volume/weight ratio differently inresponse to the same conditions, or in response to different conditionswould permit the fluid designer greater flexibility.

It will also be appreciated that the variable density fluids may be usedin operations other than wellbore operations, as in the recovery ofhydrocarbons from subterranean formations. For instance, it is expectedthat these variable density fluids would find utility in cementing orsealing, as drilling fluids or muds, as packer fluids, as workoverfluids, as completion fluids, as drill-in fluids, or in applicationswhere the variable density may affect the buoyancy of a body.

In one non-limiting embodiment herein, the elements described hereincould be used to advantage in a pseudo-gas lift operation. Generally,gas lifts are artificial-lift methods in which gas is injected into theproduction tubing to reduce the hydrostatic pressure of the fluidcolumn. The resulting reduction in bottomhole pressure allows thereservoir liquids to enter the wellbore at a higher flow rate. In onenon-restrictive method, the elements would replace the gas, oroptionally be used together with the gas, to permit reservoir fluids toflow more readily.

In this non-limiting embodiment, the lift of a produced fluid isimproved by injecting into the produced fluid at a subsurface point aneffective amount of a plurality of the elements that change theirvolume/weight ratio in response to one or more of the conditionspreviously discussed. The volume/weight ratio of the elements wouldchange in response to the condition giving added lift to the producedfluid by reducing its local average density. In this productionembodiment, it may be desirable for the average particle size of theelements to be larger than about 100 microns for ease in removal fromthe produced fluid prior to further processing. In other non-limitingembodiments, the upper limits for the various average particle sizes maybe about 1000 microns, on the other hand about 500 microns,alternatively about 250 microns, and in another case about 150 microns,whereas the lower limits for these average particles sizes may be about50 microns, about 75 microns and about 100 microns.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof, and has been postulated aseffective in providing a wellbore operation fluid having variabledensity. However, it will be evident that various modifications andchanges can be made thereto without departing from the broader spirit orscope of the invention as set forth in the appended claims. Accordingly,the specification is to be regarded in an illustrative rather than arestrictive sense. For example, specific combinations of brines,additives and other components falling within the claimed parameters,but not specifically identified or tried in a particular composition,are anticipated to be within the scope of this invention.

What is claimed is:
 1. A method of producing a fluid from a wellborecomprising providing a variable density fluid in a subterraneanformation, where the variable density fluid comprises: a base fluidselected from the group consisting of a drilling fluid, a sealant and aproduced fluid, and a plurality of elements that change theirvolume/weight ratio in response to a condition selected from the groupconsisting of pressure, temperature, and chemical composition of thebase fluid, where the elements each comprise a non-deformable body andat least one cellular, compliant component, each cellular, compliantcomponent comprising at least one gas-filled void, and where theelements have an average expanded state size and an average contractedstate size, and where the volume ratio of the average expanded statesize to the average contracted state size is at least about 2.5, andconducting a wellbore operation selected from the group consisting ofconstructing a wellbore with the variable density fluid, producing thevariable density fluid from the wellbore, sealing at least a portion ofthe wellbore and combinations thereof.
 2. The method of claim 1 wherethe average particle size of the elements is about 100 microns or less.3. The method of claim 1 where the elements each comprise anon-deformable body that is pseudo-porous.
 4. The method of claim 3where the elements each comprise a pseudo-porous non-deformable bodyselected from the group consisting of tungsten carbide, silicon carbide,silicon oxide, nanocarbon structures and combinations thereof, eachelement including at least one cellular, compliant component that is aclosed-cell elastic foam.
 5. The method of claim 1 where the at leastone gas-filled void in at least one cellular, compliant componentcomprises a gas selected from the group consisting of nitrogen, air, anoble gas, and mixtures thereof.
 6. A method of constructing a wellborecomprising: drilling a wellbore using a variable density wellboreoperation fluid within the wellbore; subjecting the wellbore operationfluid to a condition where the density of the fluid is changed inresponse to the condition, the condition selected from the groupconsisting of pressure, temperature, and chemical composition of a basefluid, where the variable density wellbore operation fluid comprises:the base fluid; and a plurality of elements that change theirvolume/weight ratio in response to a condition selected from the groupconsisting of pressure, temperature, and chemical composition of thebase fluid, where the elements each comprise a non-deformable body andat least one cellular, compliant component each cellular, compliantcomponent comprising at least one gas-filled void, and where theelements have an average expanded state size and an average contractedstate size, and where the volume ratio of the average expanded statesize to the average contracted state size is at least about 2.5.
 7. Themethod of claim 6 where the wellbore has a longer hole section ascompared with a wellbore drilled with a fluid otherwise identical exceptthe elements are absent.
 8. The method of claim 6 where the averageparticle size of the elements is about 100 microns or less.
 9. Themethod of claim 6 where the elements each comprise a non-deformable bodythat is pseudo-porous.
 10. The method of claim 9 where the elements eachcomprise a pseudo-porous non-deformable body selected from the groupconsisting of tungsten carbide, silicon carbide, silicon oxide,nanocarbon structures and combinations thereof, each element includingat least one cellular, compliant component that is a closed-cell elasticfoam.
 11. The method of claim 6 where the base fluid is a sealant, andthe method further comprises sealing at least a portion of the wellbore.12. The method of claim 6 where the at least one gas-filled void in atleast one cellular, compliant component comprises a gas selected fromthe group consisting of nitrogen, air, a noble gas, and mixturesthereof.
 13. A method of improving the lift of a produced fluidcomprising: injecting into the produced fluid at a subsurface point aneffective amount of a plurality of elements that change theirvolume/weight ratio in response to a condition selected from the groupconsisting of pressure, temperature, and chemical composition of theproduced fluid to increase the lift thereof, where the elements eachcomprise a non-deformable body and at least one cellular, compliantcomponent each cellular, compliant component comprising at least onegas-filled void, and where the elements have an average expanded statesize and an average contracted state size, and where the volume ratio ofthe average expanded state size to the average contracted state size isat least about 2.5; and lifting the produced fluid above the subsurfacepoint.
 14. The method of claim 13 where the elements each comprise anon-deformable, pseudo-porous body selected from the group consisting oftungsten carbide, silicon carbide, silicon oxide, nanocarbon structuresand combinations thereof, and the at least one cellular, compliantcomponent comprises a closed-cell elastic foam having at least onegas-filled void.
 15. The method of claim 13 where an average particlesize of the elements ranges from about 100 to about 1000 microns. 16.The method of claim 13 where the average particle size of the elementsis about 100 microns or less.
 17. The method of claim 13 where the atleast one gas-filled void in at least one cellular, compliant componentcomprises a gas selected from the group consisting of nitrogen, air, anoble gas, and mixtures thereof.